Aother closeup of the Cotton within the cupport. Another thing to notice, that CAVE like Ken NOVAK, both use similuar spider designs. (Although I don not believe NOVAK used cotton as a mirror support!)
The cylinder has a beveled edge that the diagonal mirror fits into from inside; hence it supports the optic by the mirrors face to an extent. Actually this rim or bevel runs the entire circumference of the ellipse, and adds a tiny amount of obstruction to the entire system.